Who told you this? The cories are very soft water fish, the danios are primarily soft but can manage in the moderate rang, and the butterfly loach (assume this is the Hillstream Loach) the same. Raising the GH is therefore going to affect these fish a bit, depending upon how hard it gets.
BTW, the loach is another issue. This fish need a good water current from the filter, and cooler temperatures.
Back to the GH issue...the only safe way to deal with this is either use a calcareous substrate, or use rift lake cichlid salt mixes. Given the cories I would not use a calcareous substrate. As for the "salt" mix, this is not common salt (sodium chloride) but the salts of minerals primarily calcium and magnesium. Keep in mind that you need to prepare this water outside the aquarium for every water change. It is not a matter of stirring the mineral salts into the tank water. This makes water changes more involved, but that is your call. Except some of these fish are better off with the present water.
While I agree with almost everything, of course! You know
@Byron how much I respect you, and that you know so much more than I. But the cories OP has, corydoras aeneus, are one of the few species of cories that don't require very soft water. While most do of course,
@CassCats helped me to research different cory species that have a wider range for GH, since I'm in a hard water area, and love my cories.
Luckily, corydoras aeneus (Bronze cories) have a range of 36- 268ppm GH, according to Seriously Fish. Much harder than most cory species.
Since guppies have a hardness range of :
143 – 536 ppm, also according to Seriously Fish, there can be some overlap there! I've kept them together too, but my water is within the ranges for both.
@GuppyMamma91 , Seriously Fish is a very well respected source of information about the requirements of different species! Always recommend typing "Seriously Fish Guppies" or whatever species you're looking into, into google and checking what requirements they have. There's so much conflicting and confusing info available online, but Seriously Fish is thoroughly researched and give into about tank size, water parameters, whether they can live with other fish etc, really useful stuff.
Yours are this species,
@GuppyMamma91 , yours are just an albino colour mutation of the same species, so you can get bronze cories to keep them company, or other albino corydoras aeneus - should your water end up in the hardness range that's acceptable for both the guppies and the cories.
They're a very sociable species, so if your water ends up being harder than they find acceptable, might be better to return or rehome the cories. But if you keep them, and the water ends u within their range, they'd really appreciate some friends of the same species! They do best in groups of six or more, and you'll see much more natural, playful, clownish behaviour from them when in a bigger group too.
The other essential thing to know about cories is that they really need some sand substrate. Ideally, the whole tank would have a fine sand substrate (natural colours, not brilliant white or dark, dyed black), because they filter feed from sand, and gravel poses some risks to them, both in terms of damaging their delicate facial barbels, and in harbouring nasty bacteria.
In this video, you can see in slow motion how the cory is filtering through the sand for food, expelling the sand out through his gills. That's why we urge fine, smooth grained sand, so they can do this, and not risk damage to their delicate gills;
But you can also compromise, especially since it's a new tank, and merely add a fine sand "beach" to at least 1/3 - 1/2 of the tank. It can be a bit of a hassle to keep the two substrates separated, but it's possible, and means you can drop the cory food on the beach section of the tank, to allow them to express their full natural behaviours.
This compromise is how I've wound up keeping my Pygmy Corydoras, here, and it's working well enough to have them breeding like mad